1. Field of the Invention
The present invention relates to a free space optics communication apparatus and a free space optics communication system which use light beams to perform communication with another apparatus installed at a remote location.
2. Description of Related Art
In a free space optics communication apparatus of the type described above, even a single light source can transmit a light signal. However, a plurality of light sources are used in many cases to increase transmission power in order to seek resistance to attenuation due to rain or fog in consideration of the function of propagating the light signal through the free space.
FIG. 5A shows a front view and a side view of a free space optics communication apparatus 40 which has a plurality of light sources. In FIG. 5A, reference numerals 43a to 43d show light sources, and reference numerals 41a to 41d show transmission optical systems which condense light emitted from the light sources 43a to 43d, respectively, and including information to be transmitted, form the light into beams, and send the beams toward another apparatus (not shown).
Reference numeral 42 shows a reception optical system. Light from another apparatus is condensed through the reception optical system 42 to a light signal detection element 44 such as an APD and a Pin-Photodiode to receive the light signal.
For the light sources 43a to 43d, a semiconductor laser is often used since it enables fast modulation, high output power, easy coupling of output light to an optical system, and the like.
As shown in FIG. 6, an irradiation pattern 61 of a semiconductor laser (a semiconductor laser diode chip) 60 has a generally elliptical shape in which Gaussian intensity distributions 61a and 61b are included in a longer diameter (the major axis) direction and a shorter diameter (the minor axis) direction, respectively. The Gaussian intensity distribution 61b in the shorter diameter direction is more compressed than the Gaussian intensity distribution 61a in the longer diameter direction.
In the free space optics communication apparatus 40 which uses a plurality of the semiconductor lasers as the light sources 43a to 43d, the transmission optical systems 41a to 41d have optical axes set in parallel with one another to allow another apparatus at a remote location to efficiently receive all light beams. As a result, as shown in FIG. 5B, the irradiation patterns of the respective light beams at a light-receiving unit of the other apparatus overlap one another for the most part with the centers of the patterns separated from one another corresponding to the spacings between the optical axes of the transmission optical systems 41a to 41d. 
When a combined irradiation pattern of the plurality of light beams at the light-receiving unit of the other apparatus has a generally elliptical shape as shown in FIG. 5B, the pattern has a smaller width in a shorter diameter direction.
Especially when swinging due to wind pressure or vibrations, distortion due to temperature changes, or angle variations due to changes over time occur in a building or a base on which the free space optics communication apparatus is installed, the light beam from the apparatus is likely to arrive at the light-receiving unit of the other apparatus with a shift or a deviation in a shorter diameter direction. Thus, the light-receiving unit receives a reduced amount of light or, in a worse case, no light, so that stable communication is difficult to achieve.